A biventricular pacemaker is a type of cardiac pacemaker that can pace both the right and the left ventricle (typically the lateral wall of the left ventricle). By pacing both right and left ventricles, the pacemaker is able to resynchronize a heart whose opposing walls and right and left ventricles do not contract in synchrony. Biventricular pacemakers have at least two leads, one in the right ventricle to stimulate the septum, and the other inserted through the coronary sinus to pace the lateral wall of the left ventricle. There is typically also a lead in the right atrium to facilitate synchrony with atria contraction. The use of a biventricular pacemaker is generally referred to as cardiac resynchronization therapy (CRT).
Programmable biventricular pacemakers enable optimization of the various time delays between pacemaker timing pulses. This optimization procedure requires the physician or nurse to set delays between the various timing pulses. Its general purpose is to coordinate contraction of the various chambers in the heart to improve overall efficiency and function. The onset of electrical cardiac activity in an electrocardiogram is marked by the onset of the QRS complex and corresponds to the initial impulse time (To) for the contracting ventricle. The time from the onset of the Q-wave to the closure of the mitral valve is termed electromechanical delay (EMD). The isovolumetric contraction time interval (IVCT) begins when the mitral valve closes and ends when the blood pressure within the left ventricle is sufficient to open the aortic valve. The combination of EMD and IVCT is referred to in the art as the pre-ejection time interval (PET), and is a particularly useful parameter for CRT optimization. Typically, the attending physician will want to minimize PET.
One method of optimizing settings in programmable cardiac pacemakers is disclosed in U.S. Pat. No. 8,112,150, entitled “Optimization of Pacemaker Settings”, incorporated herein by reference and assigned to the Assignee of the present application, AtCor Medical Pty. Ltd. The invention in Assignee's '150 patent uses simultaneous measurement of a patient's electrocardiogram (ECG) and a patient's peripheral blood pressure waveform in order to calculate, in real-time and non-invasively, a value correlated to the pre-ejection time (PET) for the patient's left ventricle. This value is termed a surrogate pre-ejection time (SPET) and its calculation and display enables a physician or nurse to quickly optimize PET by adjusting the programmable settings for the implanted pacemaker. To be more specific, in the system disclosed in the '150 patent, the electrocardiogram is analyzed for each pulse to determine the exact time (T0) corresponding to the onset of the QRS complex or, alternatively, the time that the Q-wave reaches its minimum value as an approximation to the onset of the QRS complex. The system also measures the patient's radial pressure pulse using for example a tonometer at the wrist. The opening of the aortic valve is marked by an abrupt rise of pressure in the aorta which results in a pressure pulse waveform rising to a peak systolic pressure and then declining. The arrival of the foot of the pressure waveform at the peripheral artery, e.g. a radial artery, is delayed by a transit time (K) for the pressure wave to travel from the aorta to the peripheral artery. For any individual patient, the travel distance for the pressure wave from the aorta to the peripheral location remains constant when the patient is at rest during the CRT optimization session, as long as the peripheral pressure is measured at a fixed location (e.g. at a fixed location on the user's wrist to the measure the pressure waveform at the radial artery). As noted in the '150 patent, testing indicates that the assumption that the pulse wave velocity for the patient remains constant over the time frame required for CRT optimization is quite accurate as long as the patient remains at rest. In the '150 patent, the time interval between Q-wave (T0) in the electrocardiogram and the foot (T2) of the peripheral pressure wave, when the ECG trace and radial waveform are measured simultaneously, represents the actual pre-ejection time interval (PET) plus a fixed value (K), which are combined as described in the '150 patent to calculate a surrogate pre-ejection time (SPET). Since there is a constant offset (K) between the actual PET and the surrogate SPET, the doctor or nurse can optimize the pacemaker settings to minimize the actual pre-ejection time PET by minimizing the surrogate SPET.